Note: Descriptions are shown in the official language in which they were submitted.
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METHOD FOR PRODUCING IMPROVED
HEAT TRANSFER SURFACE
SPECIFICATION
Improved heat transfer tube technology in recent
years has been highly dependent upon the improvement of two
phase heat transfer, that is the transfer of thermal energy
due to the phase transformation from the liquid to the vapor
phase. The methods to improve this two phase heat transfer
include both passive and active techniques. Passive techniques
include surface treatments, roughening the surface, extending
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the surfaces, swirl flow techniques, altcrnation of surface
tension, and the inclusion of additives to the coolant. Active
techniques include mechanical aids, surface vibration, fluid
vibration, and the addition of electrostatic fields.
In the area of treated surfaces, various materials
are deposited on the heat transfer tube surfaces to promote
boiling. Such materials have included Teflon, tube surface
oxides, and the addition of high surface copper powder. These
surface treatments improve the wettability of the surface and
result in a low wall super heat which eliminates boiling curve
hysteresis.
Surface roughening is a technique to provide a large
number of nucleation sites on the tube surfaces. The technique
involves the mechanical deformation of the surface to provide
¦ a large number of re-entrant cavities.
Extended surface tubes are produced by finning techniques
which yield high external surface areas to the tube and allow
very large heat transfer rates if the base temperature is in the
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film boiling range; however, nucleate boiling is not promoted
with this type of heat transfer tube.
Surface tension devices operate on the wicking principle
which relies on capillary forces while the addition of additives
to the coolant affects the wettability of the coolant to the
heat transfer tube.
A number of mechanical boiling aids have been proposed
including rotating of the boilers themselves, the introduction
of rotating plates, and the introduction of bubbles into the
vicinity of the heat surface.
The purpose of vibrating either the fluid or the sur-
face is to form localized nucleate boiling sites due to pressure
variations in the liquid. The use of electrostatic fields im-
proves mixing within the coolant and is used principally with
poorly conductingor dielectric fluids.
Of the above techniques, those that promote nucleate
boiling are of principal interest from a technological viewpoint.
The parameters of importance in a nucleate boiling tube-coolant
- system include the specific heat of the liquid, the specific
heat of the tube material, the heat transfer coefficient, the
latent heat of vaporization, the thermal conductivity of the
liquid and the heater tube, the geometry of the nucleation site,
the temperature of the coolant, vapor, and surface, the liquid
viscosity, the surface tension, and the densities of the liquid
and vapor phase.
The nucleate boiling phenomenon involves two separate
operations. The first of these is the nucleation of the vapor
phase within the liquid while the second is the subsequent growth
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of the vapor phase to form bubbles within the liquid. It has
been postulated that improved efficiency of heat transfer can
be attained when the nucleation process does not have to be
continuously redone. This nucleation process requires a large
amount of superheating. Improved efficiency can be observed
if the thermal energy is transferred by the growth of pre-exisit-
ing vapor phase nuclei. This approach has resulted in the speci-
fication of re-entrant cavities as highly effective nucleate
boiling sites.
A number of patents have been issued whereby the sur-
face of a heat transfer tube is mechanically altered to provide
these re-entrant sites. These include U.S. Patent Numbers
3,326,283; 3,454,081; 3,566,514; 3,881,342 and 3,906,604.
While all of the above patents propose the improvement of
nucleation by the mechanical introduction of nucleation sites,
they all suffer from the common characteristic of having a
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relatively ~e~ number of nucleation sites per given area of
tubing surface. This limitation is imposed by the manufacturing
tooling required to produce the tubes, and is an inherent limi-
tation for any mechanically produced tube.
The demonstrated heat transfer capability of a tube
produced with a much higher density of nucleation sitesis covered
in U.S. Patent No. 3,384,154. This tube is of the treated sur-
face variety mentioned above where copper powder particles are
sintered to the surface of the heat exchanger tube. This pro-
vides a very high density of nucleation sites on the tube sur-
face and allows retention of the vapor phase throughout the open
pore structure of the sintered surface. This sintered surface
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tube, while an effective boiling surface and heat transfer tube,
suffers from manufacturing difficulties. The copper powder is
mixed with an organic binder and sprayed onto the tube surface
for ease of handling. The coated tube is then subjected to a
high temperature exposure. This decomposes the organic binder
and sinters the copper particles together as well as to the base
tube. The sintering temperature is stated to be about 960C.
which is about 100C. below the melting point of copper. This
temperature treatment is not only difficult to do but can result
in serious degradation of the mechanical properties of the base
tube. The degradation problems can be minimized by utilizing
alloys whose superior recrystallization and grain growth char-
acteristics will reduce the amount of property degradation but
such alloys introduce added cost and have lower thermal con-
ductivity.
U.S. Patent 4,018,264 discloses a tube with improved
nucleate boiling performance as compared to a standard finned
tube which is made by initially plating the tube at high current
density to form spaced dendrites or nodules which are then
further plated at lower current densities and physically deformed.
It is among the objects of the present invention to
provide an improved heat transfer-surface and a method of making
same which will produce a very high density of nucleation sites
at a relatively low cost and without affecting the properties
of the base tube. These and other objects are achieved by
the present invention which provides a method of providing a
metal heat transfer member with a porous nucleate boiling surface
comprising the steps of applying a layer of open cell reticulated
organic foam material having an adherent coating of conductive
graphite to the surface of the metal member and electroplating
the exposed graphite coated surfaces of the reticulated foam
material and the exposed surfaces of the metal member which
underlie the foam material with a metal so as to form a
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reticulated metal surface having open cells which overlies the
surface of the metal member and is firmly adhered thereto.
The foam material can be in the form of a thin strip or
tape that is spirally wound around the base tube or it can be
in a tubular shape which could be slipped over the tube. The
foam coating can also be directly applied to the tube surface
if it is foamed in such a manner as to leave open cells rather
than a closed cell skin in contact with the base tube. The open
celled nature of the foam allows free and easy excess of the
coolant all the way to the tube surface and is more resistant
to having its nucleation sites blocked by foreign objects in
the plating solution than would be the case for a sintered
surface tube.
The reticulated foam comprises a substrate upon which
copper is plated after the foam has been made conductive. The
initial step is to apply a graphite coating to the foam which
will adhere sufficiently well to make the surface of the organic
foam electrically conductive. Standard electroplating of copper
is then used to plate the coated foam and the portions of the
tube which are not contacted by the foam and to build the thick
ness of the copper coating up to the point where it has
structural integrity. After plating, the organic precursor can
be pyrolyzed if desired.
In making experimental tubes we used a reticulated
poly~rethane foam sold as Scott Industrial Foam by Scott Paper
Company and having a 97~ void volume with a pore size con-
trolled at 39 pores per linear cm. For the experimental tubes,
strips approximately 2,54 cm wide by 1.66 mm thickness were
wrapped in a spiral fashion along the length of the base tube
and mechanically held in place by an elastic band during the
plating
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operations. The foam was previously made conductive by drawing
it through a container of finely divided graphite powder and
then passing it through the roll nip of a shear mill wherein
the different surface speeds of rotation of the work engaging
rollers forced the graphite particles into relatively firm con-
tact with the reticulated foam structure. A graphite powder
having a mesh size of less than 200 sold by the Joseph Dixon
Crucible Co. of Jersey City, New Jersey, under the formula number
8485 performed satisfactorily. The tube was then electroplated
in an air agitated standard copper sulfate electroplating solution
using a copper electrode and a DC voltage. Electroplating was
continued until a sufficiently thick copper electrodeposit was
formed so that the foam had sufficient strength to allow normal
handling. The plating conditions were 1.65 volts and 10.0 amps
for 142 minutes, resulting in a copper electrodeposit of 24.17 g
for the 30.5 cm long sample tube. Measurement of the plating
thickness is extremely difficult but the thickness appeared to
be about 10.2 to 15.2/~m.
Heat transfer testing of an as-plated tube in Refrigerant
R-ll showed a considerable improvement in the surface nucleation
boiling characteristics of this tube as compared to a standard
fin tube. The boiling characteristics were also superior to
a commercially available nucleate boiling tube produced by
mechanical means in accordance with the aforementioned U.S.
Patent No. 3,906,604. Observation of the surface boiling charac-
teristics when compared with a length of tubing as produced in
accordance with the aforementioned U.S. Patent No. 3,384,154
showed that nucleation on the foam surface was quite close to
that produced by the sintered copper surface.
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The efEect of pyrolysis of the polyurethane foam on
surface structure and boiling characteristics was then
determined. The plated foamed tube was held in a labora-
tory gas flame until pyrolysis of the graphite coated poly-
urethane substrate was complete. Optical and scanning
electron microscopy of the remaining copper foam showed a
series of very small pores along the surfaces of the
skeletal copper remaining after the pyrolysis of the sub-
strate. These pores varied in size with a maximum of about
50~ m in their largest dimension. The pores were probably
produced by the pressure of the gases created during the
pyrolysis of the organic substrate which fracture the thin
plated walls which encapsulate the organic substrate.
Boiling tests of the pyrolyzed tube in the same R-ll
coolan,t as used previously indicated superior performance
of the pyrolyzed tube as compared to the tube before pyroly-
sis. This is undoubtedly due to the large number of very
tiny vapor phase nucleation sites resulting from the poro-
sity due to the pyrolysis. Since the polyurethane can be
pyrolyzed at temperatures in the range of 302 to 482C. it
is obvious that the degradation problems which can take
place at temperatures closer to the melting point of copper
are of little consequence.
BRIEF DESCRIPTION OF THE _RAWINGS
FIG. 1 is a perspective view showing a thin strip of
reticulated foam being wound about a plain tube;
FIG. 2 is a side view illustrating the application of
graphite particles to a foam strip and the passage of the
strip through a shear mill;
FIG. 3 is a side sectional view showing the tube of
FIG. 1 being electroplated;
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FIG. 4 is a side yiew of a tube which has been
wrapped with foam and then plated being passed over a
flame to pyrolyze the foam;
FIG. 5 is a side view showing the pyrolyzed tube
of FIG. 4 having its plated surface compressed in a swag-
ing die.
FIG. 6 is a 100 x SEM photograph showing the tube
after plating; and
FIG. 7 is a 100 x SEM photograph showing the tube
of FIG. 4 after being swaged.
DESCRIPTION of the PREFERRED EMBODIMENT
. . .
Referring to Fig. 1, a tube 10, preferably-of copper,
is shown. The first step in providing the tube lQ with an
improved nucleate boiling surface is illustrated and-com-
prises the wrapping of a thin strip of reticulated poly-
urethane foam 12 about the tube 10 and anchoring it thereto
such as by means
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of a rubber band 14. The normally non-conductive foam strip
12 is preferably precoated with graphite particles 18 in order
that its surface will be conductive.
The graphite particles 18 may be applied to the foam
S strip 12 in any suitable manner. One example of a suitable
apparatus is shown in Fig. 2 where the foam strip 12 is drawn
lor.gitudinally through a shear mill 20 after graphite particles
18 have been dropped upon it from a supply hopper 24. Excess
particles fall through the foam strip 12 into a collection tray
26 from which they are recirculated to the storage hopper 24
by means of a blower 28 and a tube 30. The shear mill 20 in-
cludes a pair of rolls 34, 36 which are of like diameters but
which rotate at different speeds so as to exert a shearing
action on the foam strip 12, thus causing the particles 18 to
become attached thereto. The ratio of the speeds of surface
rotation of the lower faster roll 34 to the higher slower roll
36 are approximately 3:4.
Fig. 3 illustrates a side section of a plating appa-
ratus 40 in which the foam wrapped tube of Fig. 1 can be plated.
The plating apparatus 40 preferably comprises a vertical plating
tank or container 41 which contains a conventional copper plating
solution 42, such as one consisting of copper sulfate, sulfuric
acid and water. The tube 10 and its conductive foam layer 12
comprise the cathode of the plating apparatus while the anode
may comprise a copper tube 44 of larger diameter which surrounds
the tube 10 and is preferably evenly spaced from it. The tube 10
is shown as being mounted on a mounting block 48 of plastic or
other non-conductive material. The mounting block 48 preferably
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includes internal passages 50 and is sealed relative to the tube
10 by an O-ring 52. An air inlet tube 56 mounted in a stopper
58 in the upper end of the tube 10 permits air from a suitable
source to be carried to the bottom of the tube 10 so that it
can pass through the air passages 50 and bubble up in the form
of bubbles 60 through the solution 42 in the region between the
tube 10 and the tubular cathode 44. The bubbles 60 agitate
the plating solution 42 and provide a more uniform plating.
The cathode or tube member 10 is connected by a lead member 62
and a connector or clamp ring 64 to a battery or other DC current
source 68. The anode 44 is connected with a lead member 66 to
the battery 68.
Fig. 4 illustrates the step of pyrolyzing the organic
foam after it has been plated with a copper surface 72 in the
plating apparatus 40. The pyrolysis operation removes the foam
but leaves open spaces underneath the copper plating which
form pores under the copper surface 72.
Fig. 5 illustrates a preferred step whereby the py-
rolyzed tube is passed through a set of swaging dyes 76 to force
down the copper surface 72 so that it achieves a smaller outer
diameter 72'. The swaged surface provides the advantage of a
smaller outer diameter so that tubes can be spaced closer to-
gether in a tube bundle.
Figs. 6 and 7 are approximately lOOx (SEM) scanning
electron microscope photographs showing the surface of an ex-
perimental tube produced in the apparatus of Fig. 3. In Fig.
6, the tube 10 is shown after pyrolysis and illustrates the
porous nature of the plated copper surface 72. Fig. 7 illustrates
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a portion of the same tube after swaging through a 16.3 mm
diameter die 76 in the manner illustrated in Fig. 5. The
swaged plated copper surface 72' is compressed so that fewer
pores are visible than in Fig. 6.
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